This indicator, derived from the network analysis, measures the operational input required to provide the service levels across the system (at the minimum service standard). The number of vehicles for each mode that are in simultaneous revenue service during the weekday inter-peak period is counted. The index is expressed relative to metropolitan population (vehicles or train sets per 100,000 residents). Note that the figures for the actual numbers of vehicles required by the operators are higher as no provision is made for service breaks at the termini, contingencies for delays or disruptions, non-revenue journeys, and for vehicles undergoing scheduled or unscheduled maintenance. Greater numbers of vehicles are also generally required to operate peak hour services. Curtis C and Scheurer J (forthcoming 2016) Planning for Public Transport Accessibility, Ashgate: Aldershot [ISBN 978-1-4724-4724-1] http://www.ashgate.com/default.aspx?page=637&title_id=1219171035&edition_id=1219185914&calcTitle=1